Drugs of abuse produce strong positive reinforcing effects which are mediated, in part, by the ventral tegmental area (VTA) dopaminergic (DA) neurons. Rats, for example, will self-administer heroin or morphine directly into the VTA and systemically administered opioid- or DA-antagonists will attenuate this behavior. Microiontophoretic application of morphine directly onto VTA DA neurons produces excitation which is blocked by the opioid antagonist naloxone. Because these neurons are an important part of the brain's endogenous reward system, drugs which activate them (such as opiates, stimulants, alcohol, and nicotine) possess a high abuse potential. The present grant will utilize the following techniques to characterize the structural properties of the endogenous opioid systems which regulate VTA DA neuronal output in the rat: (1) light microscopic immunohistochemical staining techniques will be used to determine the location of the axon terminals of the three opioid peptide-containing neurons (enkephalin, beta-endorphin and dynorphin) within the 3-dimensional space of the VTA; (2) electron microscopic immunohistochemical staining techniques will be used to determine whether the opioid synaptic contacts are made with DA and/or non-DA neurons within the VTA; (3) retrograde neuroanatomical tracing and in situ hybridization techniques will be used together to determine the location of the opioid peptide-containing somata which innervate the VTA; and (4) quantitative receptor autoradiography techniques will be used to determine the location of the opioid receptor subtypes (mu delta, kappa) within the 3-dimensional space of the VTA, and their numbers and affinities. Elucidation of the structural relationships between the opioid and VTA DA neurons will provide an important step toward understanding the neuronal circuits which make up the brain's endogenous reward mechanism/s, and provide a foundation for rational drug abuse treatment approaches.